Room: Exhibit Hall | Forum 9
Purpose: Image guided radiotherapy including fiducial markers assures accurate setup and consistent target coverage in abdominal SBRT. However the cumulative dose to a deformable mobile OAR such as bowel or stomach requires independent evaluation for each treatment fraction. This work presents a comprehensive procedure with several innovative features to accurately compute the individual fraction and cumulative OAR dose useful in adaptive strategies.
Methods: Daily cone beams CBCT are converted into synthetic images CBCTsynthetic using a combination of the on-line treatment rigid and deformable image registration with the treatment planning data set CTplan to avoid errors due to incomplete data, excessive noise, and artifacts. The daily treatment dose is recomputed on CBCTsynthetic and new user-guided daily OAR contours can be generated on blended CBCTsynthetic - CBCT images for increased accuracy, a feature lacking in most published procedures. A sequence of the inverse CBCTsynthetic registration followed by limited OAR structure-based deformation assures an accurate daily OAR dose calculation back onto original data set CTplan2 for each fraction which can then be easily summed for a cumulative dose. To detect any unwanted deviations absolute volume OAR DVHs are compared for the CBCT - CBCTsynthetic and CBCTsynthetic - CTplan2 data sets.
Results: Unlike commercial products with proprietary algorithms or scattered vendor references the detailed procedure presents a complete documented general approach. We have implemented this procedure using the Varian Eclipse treatment planning system integrated with Velocity for rigid, deformable, and structure-based image registration. Systems that contain a versatile planning-registration combination lend themselves to scripting automation which facilitates rapid and consistent application.
Conclusion: Detailed procedures facilitate cumulative dose calculation for deformable organs at risk. The high doses, small number of treatment fractions, and hazard to organs at risk in abdominal SBRT mandate careful accurate evaluation.